The present invention relates to magnetic random access memory, and more specifically, to materials optimized for spin-torque memory having a seed layer and free magnetic layer for perpendicular anisotropy.
A spin torque magnetic random access memory (MRAM) device uses a two terminal spin-torque based memory element including a pinned layer, a tunnel barrier layer and a free layer in a magnetic tunnel junction (MTJ) stack as shown in FIGS. 1 and 2. For example, in FIG. 1, a MTJ stack 10 includes a pinned layer 12, a tunnel barrier layer 14 and a free magnetic layer 16. The magnetization of the pinned layer 12 is fixed in a direction (e.g., in a horizontal direction to the right as shown in FIG. 1 or in a vertically direction pointing up as shown in FIG. 2). The current passing up through the MTJ stack 10 makes the free magnetic layer 16 parallel to pinned layer 12, while a current passing down through the MTJ stack 10 makes the free magnetic layer 16 anti-parallel to the pinned layer 12. A smaller current (of either polarity) is used to read the resistance of the device, which depends on the relative orientations of the free layer 16 and pinned layer 12. As shown in FIG. 1, the free magnetic layer 16 and pinned layer 12 have their magnetizations lie in the plane which may lead to high switching currents.
FIG. 2 is an example of a MTJ stack 20 that includes a pinned layer 22, a tunnel barrier layer 24 and a free magnetic layer 26 where the magnetizations of the free magnetic layer 26 and pinned layer 22 are perpendicular to the plane. The layers 22 and 26 have perpendicular magnetic anisotropy (PMA). Some problems associated with the MTJ stack 20 include that there is a minimal number of magnetic materials with perpendicular anisotropy, and these materials have fundamental problems for use in spin-torque MRAM devices. For example, some materials have low magnetoresistance (MR) with a tunnel barrier layer of the MTJ stack and others have to be grown at high temperatures e.g., 500 C.